This invention relates to an audio amplifier circuit, and, more specifically, to protection of a loudspeaker connected to the audio amplifier circuit.
In an audio circuit with a power amplifier circuit the output of which is directly connected with a loudspeaker, some abnormal operation on the amplifier side may apply DC overvoltage directly to the loudspeaker, thereby damaging the loudspeaker. Therefore, such circuit is usually provided with a speaker protection circuit. For this protection circuit, there has conventionally been known such one as shown in FIG. 1, for example. In a power amplifier circuit A of FIG. 1, power transistors 2 and 3 driven by a driver 1 are connected in single-ended push-pull relation and, a loudspeaker 7 is directly connected between output terminals 4a and 4b (ground side) of the power amplifier via normally closed contacts 6a and 6b of a relay 5. If an abnormal operation of a power amplifier circuit A causes some DC overvoltage to develop between the output terminals 4a and 4b, a detection circuit 8 will detect this overvoltage to conduct a transistor 9, whereby a current will flow through an electromagnetic coil 10 of the relay 5 to open the contacts 6a and 6b. As a result, the loudspeaker 7 is disconnected from the output terminal 4a, and thus protected from the DC overvoltage. In the abovementioned circuit, however, if supply voltages V.sub.CC and V.sub.EE are high, the voltage V.sub.CC or V.sub.EE will be developed between the output terminals 4a and 4b due to breakdown of the power transistor 2 or 3, with the result that an arc will occurs between the contacts 6a and 6b to prevent the loudspeaker 7 from being disconnected from the output terminal 4a, thereby damaging the loudspeaker 7 by means of overvoltage.